Fix type expected by stubgenc

[henryiii]

In investigating a crash when running on a PyBind11 module, I noticed a mismatch in the types; infer_prop_type_from_docstring can clearly get None, but it has a type of plain str. Fix the annotation to match the expectation. Now, when rebuilding Mypy, the error goes away and this now works without crashing, and can completely generate stubs.

Here's one of the uses where None is obviously possible:

mypy/mypy/stubgenc.py

Lines 220 to 221 in d104886

	docstr = getattr(obj, '__doc__', None)
	inferred = infer_prop_type_from_docstring(docstr)

Fixes #7692.

Example printout when this error is seen:

    sys.exit(main())
  File "mypy/stubgen.py", line 1564, in main
  File "mypy/stubgen.py", line 1457, in generate_stubs
  File "mypy/stubgenc.py", line 51, in generate_stub_for_c_module
  File "mypy/stubgenc.py", line 273, in generate_c_type_stub
  File "mypy/stubgenc.py", line 221, in generate_c_property_stub
TypeError: str object expected; got None

[JelleZijlstra]

Looks good. I wonder why mypy itself didn't catch that—are we running with strict-optional off perhaps?

[t-fi]

Hi, will this PR be merged? It looks quite simple and finished, or is there some work to be done still? I am interested since I have this issue when running stubgen on several different modules.

For anyone interested, you can install the fixed version via:
python3 -m pip install -U git+git://github.com/henryiii/mypy.git@7f4dc937d338875104b0cd2f19d8f06d24713e55

I would not recommend installing some strangers repo on any machine. You can install that version once, generate stubs, and then remove it again :)

[JukkaL]

Looks good, thanks for the PR!